1. Field of the Invention
The invention relates to a method and an apparatus for recovering a sterilizing gas, in particular ethylene oxide.
2. Description of the Prior Art
For sterilizing medical equipment, including packaged equipment, it is usual to expose the materials to be sterilized in a sterilizing chamber to a sterilizing gas, usually an alkylene oxide. The ethylene oxide generally employed is a very toxic gas which in combination with air is readily inflammable and explosive, with a lower explosion limit of 2.6% and an upper explosion limit of 100% ethylene oxide content in an ethylene oxide/air mixture. Thus, even without addition of air pure ethylene oxide (ETO) can explode in the presence of a suitable ignition source. To reduce the risks involved therewith, the established practice is to dilute ETO with a relatively inert gas. Typical is a mixture of 12% by weight ETO with 88% by weight Freon R 12 (dichlorodifluoromethane).
To avoid having to release the toxically acting and relatively expensive constituents of such a mixture into the environment after each sterilizing cycle, in recent years methods and apparatuses have been developed for recovering the sterilizing gas and the inert additive gases mentioned. Thus, U.S. Pat. No. 3,549,312 discloses a method and an apparatus for recovering alkylene oxide and the admixed inert constituents in which the mixture of alkylene oxide and the inert additives extracted from the sterilizing chamber by suction is supplied to a precooler and thereafter to an adsorber. The mixture freed from moisture in the adsorber then passes to a condenser for liquefying the alkylene oxide and the admixed Freon R 12 and finally to a reservoir for intermediate storage of the liquid mixture. Finally, this mixture can be supplied to the sterilizing chamber again from the reservoir subjected to excess pressure. Although this publication does refer to the use of pure ethylene oxide as sterilizing gas, a method or an apparatus operating according to this teaching will no doubt be limited to the use of a mixture of ethylene oxide and a relatively inert gas because no steps are apparent for countering the problems of the easy inflammability of pure ethylene oxide and avoiding excess pressure in the sterilizing chamber and the following recovery. Excess pressure can however lead to the escape of toxic gases to the environment.
Further methods and apparatuses for recovering a sterilizing gas or a mixture of a sterilizing gas and usually a gaseous fluorinated hydrocarbon are known from EP-A-0 130 319, U.S. Pat. No. 3,989,461 and EP-A-0 326 985. All the methods and apparatuses disclosed therein operate at least at times and in some areas of such apparatuses at pressures above the ambient pressure, thereby resulting in a continuous danger of leakage losses. Although the possibility of using pure ethylene oxide as sterilizing gas is mentioned in EP-A-0 130 319, the particular problems in the handling thereof and in particular their technical solution are not mentioned. They were obviously not recognized.